The naturally occurring eight compounds with vitamin E activity are derivatives of 6-chromanol (Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 27 (1996), VCH Verlagsgesellschaft, Chapter 4., 478-488, Vitamin E). The tocopherol group (1a-d) has a saturated side chain, while the tocotrienol group (2a-d) has an unsaturated side chain:
    1a, α-tocopherol: R1=R2=R3=CH3     1b, β-tocopherol: R1=R3=CH3, R2=H    1c, γ-tocopherol: R1=H, R2=R3=CH3     1d, δ-tocopherol: R1=R2=H, R3=CH3 
    2a, α-tocotrienol: R1=R2=R3=CH3     2b, β-tocotrienol: R1=R3=CH3, R2=H    3c, γ-tocotrienol: R1=H, R2=R3=CH3     4d, δ-tocotrienol: R1=R2=H, R3=CH3 
Within the present invention, vitamin E is understood to include all of the abovementioned tocopherols and tocotrienols with vitamin E activity.
These compounds with vitamin E activity are important natural lipid-soluble and solid antioxidants. Vitamin E deficiency leads to pathophysiological situations in humans and animals. Vitamin E compounds are thus of great economic value as additives in the food-and-feed sector, in pharmaceutical formulations and in cosmetic applications.
An economical process for the production of vitamin E compounds and of foods and feeds with an increased vitamin E content are therefore of great importance.
Particularly economical processes are biotechnological processes which exploit natural vitamin E-producing organisms or vitamin E-producing organisms which have been optimized by genetic modification.
FIG. 62 shows a biosynthesis scheme of α-tocopherol in higher plants.
In higher plants, tyrosine is formed starting from chorismate via prephenate and arogenate. The aromatic amino acid tyrosine is converted into hydroxyphenylpyruvate by the enzyme tyrosine aminotransferase, and hydroxyphenylpyruvate is converted into homogentisic acid by dioxygenation.
Homogentisic acid is subsequently bound to phytyl pyrophosphate (PPP) or geranylgeranyl pyrophosphate in order to form the precursors of α-tocopherol and α-tocotrienol, namely 2-methyl-6-phytylhydroquinone and 2-methyl-6-geranylgeranylhydroquinone. Methylation steps with S-adenosylmethionine as methyl group donor first lead to 2,3-dimethyl-6-phytylquinone, subsequent cyclization leads to γ-tocopherol and further methylation leads to α-tocopherol.
Experiments to increase the metabolite flux in order to increase the tocopherol or tocotrienol content in transgenic organisms by overexpressing individual biosynthesis genes are known.
WO 97/27285 describes a modification of the tocopherol content by increased expression or by downregulation of the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD).
WO 99/04622 and D. DellaPenna et al., Science 1998, 282, 2098-2100 describe gene sequences encoding a γ-tocopherol methyltransferase from Synechocystis PCC6803 and Arabidopsis thaliana and their incorporation into transgenic plants which have a modified vitamin E content.
WO 99/23231 shows that the expression of a geranylgeranyl reductase in transgenic plants results in an increased tocopherol biosynthesis.
WO 00/08169 describes gene sequences encoding a 1-deoxy-D-xylose-5-phosphate synthase and a geranylgeranyl-pyrophosphate oxidoreductase and their incorporation into transgenic plants which have a modified vitamin E content.
WO 00/68393 and WO 00/63391 describe gene sequences encoding a phytyl/prenyl transferase and their incorporation into transgenic plants which have a modified vitamin E content.
WO 00/61771 postulates that the combination of a sterol metabolism gene in combination with a tocopherol metabolism gene can lead to an increased tocopherol content in transgenic plants.
Arabidopsis thaliana genes which are inducible by the phytotoxin coronatine are disclosed in a PhD thesis by A. Lopoukhina (Characterization of coronatine regulated genes from Arabidopsis thaliana, PhD thesis at the Ruhr-Universität Bochum, Department of Plant Physiology, 1999) and in a poster contribution by H. Holländer-Czytko et al. at the “Botanikertagung 2000”, Jena, 17-22.9.2000. In one of these genes, the derived amino acid sequence shows approximately 35% homology with known tyrosine aminotransferases. A low degree of enzyme activity of a tyrosine aminotransferase was detected by heterologous expression of the putative tyrosine aminotransferase gene in E. coli. It is disclosed that the treatment of plants with coronatine and the wounding of plants lead to an accumulation of the putative tyrosine aminotransferase-specific mRNA, the putative tyrosine aminotransferase and the measurable enzyme activity. Page 72 et seq. of the PhD thesis furthermore disclose that the wounding of plants is known to lead to the formation of reactive oxygen species which are scavenged by antioxidative compounds such as tocopherol, carotenoids or rosmaric acid.
While all of these methods, with the exception of the last-mentioned prior art, give rise to genetically modified organisms, in particular plants, which, as a rule, have a modified vitamin E content, they have the disadvantage that the level of the vitamin E content in the prior-art genetically modified organisms is as yet unsatisfactory.
It was therefore an object of the present invention to provide a further process for the production of vitamin E by growing organisms, or to provide further transgenic organisms which produce vitamin E, which have optimized characteristics such as, for example, a higher vitamin E content, and which do not exhibit the above-described disadvantage of the prior art.
We have found that this object is achieved by a process for the production of vitamin E wherein organisms are grown which have an increased tyrosine aminotransferase activity over the wild type.